Porous Cobalt Phosphide Polyhedrons with Iron Doping as an Efficient Bifunctional Electrocatalyst

Iron (Fe)‐doped porous cobalt phosphide polyhedrons are designed and synthesized as an efficient bifunctional electrocatalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The synthesis strategy involves one‐step route for doping foreign metallic element and forming porous cobalt phosphide polyhedrons. With varying doping levels of Fe, the optimized Fe‐doped porous cobalt phosphide polyhedron exhibits significantly enhanced HER and OER performances, including low onset overpotentials, large current densities, as well as small Tafel slopes and good electrochemical stability during HER and OER.     

FeS2/CoS2 Interface Nanosheets as Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Electrochemical water splitting to produce hydrogen and oxygen, as an important reaction for renewable energy storage, needs highly efficient and stable catalysts. Herein, FeS₂/CoS₂ interface nanosheets (NSs) as efficient bifunctional electrocatalysts for overall water splitting are reported. The thickness and interface disordered structure with rich defects of FeS₂/CoS₂ NSs are confirmed by atomic force microscopy and high‐resolution transmission electron microscopy. Furthermore, extended X‐ray absorption fine structure spectroscopy clarifies that FeS₂/CoS₂ NSs with sulfur vacancies, which can further increase electrocatalytic performance. Benefiting from the interface nanosheets' structure with abundant defects, the FeS₂/CoS₂ NSs show remarkable hydrogen evolution reaction (HER) performance with a low overpotential of 78.2 mV at 10 mA cm⁻² and a superior stability for 80 h in 1.0 m KOH, and an overpotential of 302 mV at 100 mA cm⁻² for the oxygen evolution reaction (OER). More importantly, the FeS₂/CoS₂ NSs display excellent performance for overall water splitting with a voltage of 1.47 V to achieve current density of 10 mA cm⁻² and maintain the activity for at least 21 h. The present work highlights the importance of engineering interface nanosheets with rich defects based on transition metal dichalcogenides for boosting the HER and OER performance.     

An Efficient Cobalt Phosphide Electrocatalyst Derived from Cobalt Phosphonate Complex for All‐pH Hydrogen Evolution Reaction and Overall Water Splitting in Alkaline Solution

The development of low‐cost and highly efficient electrocatalysts via an eco‐friendly synthetic method is of great significance for future renewable energy storage and conversion systems. Herein, cobalt phosphides confined in porous P‐doped carbon materials (Co‐P@PC) are fabricated by calcinating the cobalt‐phosphonate complex formed between 1‐hydroxyethylidenediphosphonic acid and Co(NO₃)₂ in alkaline solution. The P‐containing ligand in the complex acts as the carbon source as well as in situ phosphorizing agent for the formation of cobalt phosphides and doping P element into carbon material upon calcination. The Co‐P@PC exhibits high activity for all‐pH hydrogen evolution reaction (overpotentials of 72, 85, and 76 mV in acidic, neutral, and alkaline solutions at the current density of 10 mA cm^?2) and oxygen evolution reaction in alkaline solution (an overpotential of 280 mV at the current density of 10 mA cm^?2). The alkaline electrolyzer assembled from the Co‐P@PC electrodes delivers the current density of 10 mA cm^?2 at the voltage of 1.60 V with a durability of 60 h. The excellent activity and long‐term stability of the Co‐P@PC derives from the synergistic effect between the active cobalt phosphides and the porous P‐doped carbon matrix.     

NiO as a Bifunctional Promoter for RuO2 toward Superior Overall Water Splitting

Conventional development of nanomaterials for efficient electrocatalysis is largely based on performance‐oriented trial‐and‐error/iterative approaches, while a rational design approach at the atomic/molecular level is yet to be found. Here, inspired by a fundamental understanding of the mechanism for both oxygen and hydrogen evolution half reactions (OER/HER), a unique strategy is presented to engineer RuO₂ for superior alkaline water electrolysis through coupling with NiO as an efficient bifunctional promoter. Benefitting from desired potential‐induced interfacial synergies, NiO‐derived NiOOH improves the oxygen binding energy of RuO₂ for enhanced OER, and NiO also promotes water dissociation for enhanced HER on RuO₂‐derived Ru. The resulting hybrid material exhibits remarkable bifunctional activities, affording 2.6 times higher OER activity than that of RuO₂ and an HER activity comparable to Pt/C. As a result, the simple system requires only 1.5 V to deliver 10 mA cm^?2 for overall alkaline water splitting, outperforming the benchmark PtC/NF||IrO₂/NF couple with high mass loading. Comprehensive electrochemical investigation reveals the unique and critical role of NiO on the optimized RuO₂/NiO interface for synergistically enhanced activities, which may be extended to broader (electro)catalytic systems.     

Strategic Atomic Layer Deposition and Electrospinning of Cobalt Sulfide/Nitride Composite as Efficient Bifunctional Electrocatalysts for Overall Water Splitting

Reported herein is comprehensive study of a highly active and stable cobalt catalyst for overall water splitting. This composite SFCNF/Co_1?xS@CoN, consisting of S‐doped flexible carbon nanofiber (SFCNF) matrix, Co_1?xS nanoparticles, and CoN coatings, is prepared by integration of electrospinning and atomic layer deposition (ALD) technique. Representative results include the following: 1) ultrathin CoN layer is deposited by ALD on the surface of flexible substrate without any sacrifice of SFCNF and Co_1?xS; 2) the composite exhibits strong electrocatalytic activity in both acidic and basic solutions. The overpotentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are 20 and 180 mV, respectively, at a current density of 10 mA cm^?2 in basic medium. A small Tafel slope of 54.4 mV dec^?1 is observed in 0.5 m H₂SO₄ electrolyte; 3) tested as overall water splitting electrode, the composite records a current density of 10 mA cm^?2 at a relative low cell voltage of 1.58 V and long‐term stability for 20 h at a current density of up to 50 mA cm^?2. The superior performance for overall water splitting is probably attributed to the synergistic effect of Co_1?xS and ALD CoN. Specifically, implementation of ALD can be extended to innovate nanostructured materials for overall water splitting and even other renewable energy aspects.     

IrPd Nanoalloy-Structured Bifunctional Electrocatalyst for Efficient and pH-Universal Water Splitting

The lack of high efficiency and pH-universal bifunctional electrocatalysts for water splitting to hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) hinders the large-scale production of green hydrogen. Here, an IrPd electrocatalyst supported on ketjenblack that exhibits outstanding bifunctional performance for both HER and OER at wide pH conditions is presented. The optimized IrPd catalyst exhibits a specific activity of 4.46 and 3.98 A mg_Ir⁻¹ in the overpotential of 100 and 370 mV for HER and OER, respectively, in alkaline conditions. When applied to the anion exchange membrane electrolyzer, the Ir₄₄Pd₅₆/KB catalyst shows a stability of >20 h at a current of 250 mA cm⁻² for water decomposition, indicating promising prospects for practical applications. Beyond offering an advanced electrocatalyst, this work also guides the rational design of desirable bifunctional electrocatalysts for HER and OER by regulating the microenvironments and electronic structures of metal catalytic sites for diverse catalysis.     

Constructing Pure Phase Tungsten‐Based Bimetallic Carbide Nanosheet as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Carbides are commonly regarded as efficient hydrogen evolution reaction (HER) catalysts, but their poor oxygen evolution reaction (OER) catalytic activities seriously limit their practical application in overall water splitting. Here, vertically aligned porous cobalt tungsten carbide nanosheet embedded in N‐doped carbon matrix (Co₆W₆C@NC) is successfully constructed on flexible carbon cloth (CC) as an efficient bifunctional electrocatalyst for overall water splitting via a facile metal–organic framework (MOF) derived method. The synergistic effect of Co and W atoms effectively tailors the electron state of carbide, optimizing the hydrogen‐binding energy. Thus Co₆W₆C@NC shows an enhanced HER performance with an overpotential of 59 mV at a current density of ?10 mA cm^?2. Besides, Co₆W₆C@NC easily in situ transforms into tungsten actived cobalt oxide/hydroxide during the OER process, serving as OER active species, which provides an excellent OER activity with an overpotential of 286 mV at a current density of ?10 mA cm^?2. The water splitting device, by applying Co₆W₆C@NC as both the cathode and anode, requires a low cell voltage of 1.585 V at 10 mA cm^?2 with the great stability in alkaline solution. This work provides a feasible strategy to fabricate bimetallic carbides and explores their possibility as bifunctional catalysts toward overall water splitting.     

A Cost‐Efficient Bifunctional Ultrathin Nanosheets Array for Electrochemical Overall Water Splitting

The design of cost‐efficient earth‐abundant catalysts with superior performance for the electrochemical water splitting is highly desirable. Herein, a general strategy for fabricating superior bifunctional water splitting electrodes is reported, where cost‐efficient earth‐abundant ultrathin Ni‐based nanosheets arrays are directly grown on nickel foam (NF). The newly created Ni‐based nanosheets@NF exhibit unique features of ultrathin building block, 3D hierarchical structure, and alloy effect with the optimized Ni₅Fe layered double hydroxide@NF (Ni₅Fe LDH@NF) exhibiting low overpotentials of 210 and 133 mV toward both oxygen evolution reaction and hydrogen evolution reaction at 10 mA cm^?2 in alkaline condition, respectively. More significantly, when applying as the bifunctional overall water splitting electrocatalyst, the Ni₅Fe LDH@NF shows an appealing potential of 1.59 V at 10 mA cm^?2 and also superior durability at the very high current density of 50 mA cm^?2.     

Ultrathin Porous NiFeV Ternary Layer Hydroxide Nanosheets as a Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Herein, the hydrothermal synthesis of porous ultrathin ternary NiFeV layer double hydroxides (LDHs) nanosheets grown on Nickel foam (NF) substrate as a highly efficient electrode toward overall water splitting in alkaline media is reported. The lateral size of the nanosheets is about a few hundreds of nanometers with the thickness of ≈10 nm. Among all molar ratios investigated, the Ni_0.75Fe_0.125V_0.125‐LDHs/NF electrode depicts the optimized performance. It displays an excellent catalytic activity with a modest overpotential of 231 mV for the oxygen evolution reaction (OER) and 125 mV for the hydrogen evolution reaction (HER) in 1.0 m KOH electrolyte. Its exceptional activity is further shown in its small Tafel slope of 39.4 and 62.0 mV dec^?1 for OER and HER, respectively. More importantly, remarkable durability and stability are also observed. When used for overall water splitting, the Ni_0.75Fe_0.125V_0.125‐LDHs/NF electrodes require a voltage of only 1.591 V to reach 10 mA cm^?2 in alkaline solution. These outstanding performances are mainly attributed to the synergistic effect of the ternary metal system that boosts the intrinsic catalytic activity and active surface area. This work explores a promising way to achieve the optimal inexpensive Ni‐based hydroxide electrocatalyst for overall water splitting.     

Heteroatom Doped Amorphous/Crystalline Ruthenium Oxide Nanocages as a Remarkable Bifunctional Electrocatalyst for Overall Water Splitting

Developing robust and highly active bifunctional electrocatalysts for overall water splitting is critical for efficient sustainable energy conversion. Herein, heteroatom-doped amorphous/crystalline ruthenium oxide-based hollow nanocages (M-ZnRuO_x (MCo, Ni, Fe)) through delicate control of composition and structure is reported. Among as-synthesized M-ZnRuO_x nanocages, Co-ZnRuO_x nanocages deliver an ultralow overpotential of 17 mV at 10 mA cm⁻² and a small Tafel slope of 21.61 mV dec⁻¹ for hydrogen evolution reaction (HER), surpassing the commercial Pt/C catalyst, which benefits from the synergistic coupling effect between electron regulation induced by Co doping and amorphous/crystalline heterophase structure. Moreover, the incorporation of Co prevents Ru from over-oxidation under oxygen evolution reaction (OER) operation, realizing the leap from a monofunctional to multifunctional electrocatalyst and then Co-ZnRuO_x nanocages exhibit remarkable OER catalytic activity as well as overall water splitting performance. Combining theory calculations with spectroscopy analysis reveal that Co is not only the optimal active site, increasing the number of exposed active sites while also boosting the long-term durability of catalyst by modulating the electronic structure of Ru atoms. This work opens a considerable avenue to design highly active and durable Ru-based electrocatalysts.     

A Heterostructure Coupling of Exfoliated Ni–Fe Hydroxide Nanosheet and Defective Graphene as a Bifunctional Electrocatalyst for Overall Water Splitting

Herein, the authors demonstrate a heterostructured NiFe LDH‐NS@DG10 hybrid catalyst by coupling of exfoliated Ni–Fe layered double hydroxide (LDH) nanosheet (NS) and defective graphene (DG). The catalyst has exhibited extremely high electrocatalytic activity for oxygen evolution reaction (OER) in an alkaline solution with an overpotential of 0.21 V at a current density of 10 mA cm^?2, which is comparable to the current record (≈0.20 V in Fe–Co–Ni metal‐oxide‐film system) and superior to all other non‐noble metal catalysts. Also, it possesses outstanding kinetics (Tafel slope of 52 mV dec^?1) for the reaction. Interestingly, the NiFe LDH‐NS@DG10 hybrid has also exhibited the high hydrogen evolution reaction (HER) performance in an alkaline solution (with an overpotential of 115 mV by 2 mg cm^?2 loading at a current density of 20 mA cm^?2) in contrast to barely HER activity for NiFe LDH‐NS itself. As a result, the bifunctional catalyst the authors developed can achieve a current density of 20 mA cm^?2 by a voltage of only 1.5 V, which is also a record for the overall water splitting. Density functional theory calculation reveals that the synergetic effects of highly exposed 3d transition metal atoms and carbon defects are essential for the bifunctional activity for OER and HER.     

Self‐Supportive Mesoporous Ni/Co/Fe Phosphosulfide Nanorods Derived from Novel Hydrothermal Electrodeposition as a Highly Efficient Electrocatalyst for Overall Water Splitting

Low cost and highly efficient bifuctional catalysts for overall water electrolysis have drawn considerable interests over the past several decades. Here, rationally synthesized mesoporous nanorods of nickel–cobalt–iron–sulfur–phosphorus composites are tightly self‐supported on Ni foam as a high‐performance, low cost, and stable bifunctional electrocatalyst for water electrolysis. The targeted designing and rational fabrication give rise to the nanorod‐like morphology with large surface area and excellent conductivity. The NiCoFe‐PS nanorod/NF can reach 10 mA cm^?2 at a small overpotential of 195 mV with a Tafel slope of 40.3 mV dec^?1 for the oxygen evolution reaction and 97.8 mV with 51.8 mV dec^?1 for the hydrogen evolution reaction. Thus, this bifunctional catalyst shows low potentials of 1.52 and 1.76 V at 10 and 50 mA cm^?2 toward overall water splitting with excellent stability for over 200 h, which are superior to most non‐noble metal‐based bifunctional electrocatalysts recently. This work provides a new strategy to fabricate multiple metal‐P/S composites with the mesoporous nanorod‐like structure as bifunctional catalysts for overall water splitting.     

Porous Cobalt–Nickel Hydroxide Nanosheets with Active Cobalt Ions for Overall Water Splitting

Low‐cost and high‐performance catalysts are of great significance for electrochemical water splitting. Here, it is reported that a laser‐synthesized catalyst, porous Co_0.75Ni_0.25(OH)₂ nanosheets, is highly active for catalyzing overall water splitting. The porous nanosheets exhibit low overpotentials for hydrogen evolution reaction (95 mV@10 mA cm^?2) and oxygen evolution reaction (235 mV@10 mA cm^?2). As both anode and cathode catalysts, the porous nanosheets achieve a current density of 10 mA cm^?2 at an external voltage of 1.56 V, which is much lower than that of commercial Ir/C‐Pt/C couple (1.62 V). Experimental and theoretical investigations reveal that numerous Co³⁺ ions are generated on the pore wall of nanosheets, and the unique atomic structure around Co³⁺ ions leads to appropriate electronic structure and adsorption energy of intermediates, thus accelerating hydrogen and oxygen evolution.     

In Situ Derived CoB Nanoarray: A High‐Efficiency and Durable 3D Bifunctional Electrocatalyst for Overall Alkaline Water Splitting

The development of efficient bifunctional catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of extreme importance for future renewable energy systems. This Communication reports the recent finding that room‐temperature treatment of CoO nanowire array on Ti mesh by NaBH₄ in alkaline media leads to in situ development of Co?B nanoparticles on nanowire surface. The resulting self‐supported Co?B@CoO nanoarray behaves as a 3D bifunctional electrocatalyst with high activity and durability for both HER (<17% current density degradation after 20 h electrolysis) and OER (<14% current density degradation after 20 h electrolysis) with the need of the overpotentials of 102 and 290 mV to drive 50 mA cm^?2 in 1.0 m KOH, respectively. Moreover, its two‐electrode alkaline water electrolyzer also shows remarkably high durability and only demands a cell voltage of 1.67 V to deliver 50 mA cm^?2 water‐splitting current with a current density retention of 81% after 20 h electrolysis. This work provides a promising methodology for the designing and fabricating of metal‐boride based nanoarray as a high‐active water‐splitting catalyst electrode for applications.     

Transition‐Metal‐Doped RuIr Bifunctional Nanocrystals for Overall Water Splitting in Acidic Environments

The establishment of electrocatalysts with bifunctionality for efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in acidic environments is necessary for the development of proton exchange membrane (PEM) water electrolyzers for the production of clean hydrogen fuel. RuIr alloy is considered to be a promising electrocatalyst because of its favorable OER performance and potential for HER. Here, the design of a bifunctional electrocatalyst with greatly boosted water‐splitting performance from doping RuIr alloy nanocrystals with transition metals that modify electronic structure and binding strength of reaction intermediates is reported. Significantly, Co‐RuIr results in small overpotentials of 235 mV for OER and 14 mV for HER (@ 10 mA cm^?2 current density) in 0.1 m HClO₄ media. Therefore a cell voltage of just 1.52 V is needed for overall water splitting to produce hydrogen and oxygen. More importantly, for a series of M‐RuIr (M = Co, Ni, Fe), the catalytic activity dependence at fundamental level on the chemical/valence states is used to establish a novel composition‐activity relationship. This permits new design principles for bifunctional electrocatalysts.     

Cobalt Covalent Doping in MoS2 to Induce Bifunctionality of Overall Water Splitting

The layer‐structured MoS₂ is a typical hydrogen evolution reaction (HER) electrocatalyst but it possesses poor activity for the oxygen evolution reaction (OER). In this work, a cobalt covalent doping approach capable of inducing HER and OER bifunctionality into MoS₂ for efficient overall water splitting is reported. The results demonstrate that covalently doping cobalt into MoS₂ can lead to dramatically enhanced HER activity while simultaneously inducing remarkable OER activity. The catalyst with optimal cobalt doping density can readily achieve HER and OER onset potentials of ?0.02 and 1.45 V (vs reversible hydrogen electrode (RHE)) in 1.0 m KOH. Importantly, it can deliver high current densities of 10, 100, and 200 mA cm^?2 at low HER and OER overpotentials of 48, 132, 165 mV and 260, 350, 390 mV, respectively. The reported catalyst activation approach can be adapted for bifunctionalization of other transition metal dichalcogenides.